There were 8,583 homicides by firearms in 2011, out of 12,664 homicides total, according to the FBI. This means that more than two-thirds of homicides involve a firearm. 6,220 of those homicides by firearm (72%) are known to have involved a handgun.

It's worth noting that violent crime rates of all types have been steadily decreasing since the early 1990s. No one is quite sure what is causing this decrease, though there are many theories, ranging from tighter gun control laws to more innovative policing and changes in the drug market. Whatever the cause of this decline, America still has a homicide rate of 4.7 murders per 100,000 people, which is one of the highest of all developed countries (see: international comparison).

Gun violence also affects more than its victims. In areas where it is prevalent, just the threat of violence makes neighborhoods poorer. It's very difficult to quantify the total harm caused by gun violence, but by asking many people how much they would pay to avoid this threat -- a technique called contingent valuation -- researchers have estimated a cost to American society of $100 billion dollars.

Guns are also involved in suicides and accidents. 19,392 of 38,264 suicides in 2010 involved a gun (50%), according to the CDC. There were 606 firearm-related accidents in the same year -- about 5% of the number of intentional gun deaths.

How many guns are there in the U.S.?

There are about 310 million guns in the country. About 40% of households have them, a fraction that has been slowly declining over the last few decades, down from about 50% in the 1960s. Meanwhile, the overall number of guns has increased to about one gun per person, up from one gun for every two persons in the 1960s. This means that gun ownership has gotten much more concentrated among fewer households: if you own one gun, you probably own several. America has the highest rate of gun ownership of any country in the world, by a wide margin (see: international comparison).

The FBI defines a "mass murder" as four or more murders during the same incident. This is an arbitrary number, but a dividing line is useful when asking whether there are differences between mass shootings and other kinds of gun violence. The most comprehensive public list of U.S. mass shootings is the spreadsheet of 62 incidents from 1982-2012, compiled by Mother Jones. Their list shows:

Killers used a semi-automatic handgun in 75% of incidents, which is about the same percentage as the 72% in overall gun violence.

Killers used an assault weapon in 40% of incidents. This is much higher than overall assault weapon use in crimes, estimated at less than 2%.

The guns were obtained legally in 79% of mass shootings.

Many of the shooters showed signs of mental illness, but in only two cases was there a prior diagnosis.

There were no cases where an armed civilian fired back.

2012 was the worst year in American history, in terms of total victims. A graph of yearly victims shows a slight upward trend. But the pattern is a lot less clear without the 2012 peak, and because yearly numbers vary so widely, it's likely that there will be many fewer victims next year.

Several criminologistsdeny that mass shootings are increasing. Although these incidents dominate headlines and conversation, it's important to remember that they account for only a small fraction of gun violence in the United States. For example, the spike of 72 deaths in 2012 includes only 0.8% of all firearm-related homicides in 2011 (the last year for which statistics are available.) Many gun deaths, especially in large cities, never make the news. This means that the most effective gun violence reduction strategies -- in terms of lives saved -- might not target mass shootings at all.

What gun control laws currently exist?

There are two major federal laws that regulate firearm ownership and sales. The National Firearms Act of 1934 restricts civilians from owning automatic weapons, short-barreled shotguns, hand grenades, and other powerful arms. The Gun Control Act of 1968 focuses on commerce. It prohibits mail-order sales of weapons, and requires anyone in the business of selling guns to be federally licensed and keep permanent sales records. It also prohibits knowingly selling a gun to those with prior criminal records, minors, individuals with mental health problems, and a few other categories of people.

The Brady Handgun Violence Prevention Act of 1993 requires licensed gun dealers to perform background checks. Background checks are not required for private gun sales (though, as mentioned above, it's still a crime to knowingly sell a gun to someone with a criminal record). To ensure privacy, Section 103(i) of the Act prevents the Federal government from keeping the names submitted for background checks, or using this information to create any sort of registry of gun owners.

From 1994 to 2004, the Federal Assault Weapons Ban prohibited the sale and manufacture of semi-automatic weapons (in which each pull of the trigger fires one shot) with various military features such as large-capacity magazines and pistol grips. It was still legal to keep previously owned weapons. The law expired in 2004 due to a built-in "sunset" clause.

These federal laws set minimum standards, but many states have also passed various types of gun laws. These laws determine which weapons are legal to own, and also set requirements on sales, background checks, storage, open and concealed carrying permits, and sentencing of gun-related crimes.

The firearms debate usually revolves around "gun control" -- that is, laws that would make guns harder to buy, carry, or own. But this is not the only way of reducing gun violence. It is possible to address gun use instead of availability. For example, Project Exile moved all gun possession offenses in Richmond, Virginia, to federal courts instead of state courts, where minimum sentences are longer. Policies like these, which concern gun use, are sometimes said to operate on gun "demand," as opposed to gun control laws, which affect "supply."

Similarly, while the idea of new laws gets most of the attention, some projects have focused on enforcing existing laws more effectively, or changing policing strategies the way Boston's Operation Ceasefire did in the 1990s. In fact, launching community-based programs has proven to be one of the most effective strategies for reducing gun violence. (See: What has worked, below.)

There have also been programs based on other principles, such as public safety education and gun buy-back campaigns. The White House proposals (see below) address both gun access and gun use, and include both new laws and enhanced enforcement of existing laws.

Economist John Lott did extensive work on this question in the late 1990s, culminating in his 1998 book More Guns, Less Crime. He studied the effect of right-to-carry laws by examining violent crime rates before and after they were implemented in various states, up until 1992, and concluded that such laws decreased homicides by an average of 8%. Lott's data and methods have been extensively reviewed since then. A massive 2004 report by a 16-member panel of the National Research Council found that there was not enough evidence to say either way whether right-to-carry laws affected violence. In 2010, different researchers re-examined Lott's work, the NRC report, and additional data up through 2006, and reaffirmed that there is no evidence that right-to-carry laws reduce crime.

Meanwhile, otherstudies have suggested that reduced access to guns would result in less crime. These studies compared homicide rates with gun availability in various states and cities. The most comprehensive estimate is that a 10% reduction in U.S. households with guns would result in a 3% reduction in homicides. Internationally, the effect of reductions in gun ownership might be much larger. This might have to do with the large number of guns already available in the U.S.: Any reduction in gun violence hinges on whether gun control laws would actually make it prohibitively difficult to get a gun.

In principle, it's not necessary to keep guns away from everyone, just those who would misuse them. Background checks are promising because a high fraction of future killers already have a criminal record. In one study in Illinois, 71% of those convicted of homicide had a previous arrest, and 42% had a prior felony conviction.

Also, although it's generally illegal to sell guns across state lines, in practice this is very common. There's abundant evidence that under the current system, guns flow easily between legal and illegal markets. Washington, D.C,. banned all handguns in 1976, and Chicago did the same in 1982. In neither case did the percentage of suicides using firearms -- considered a very good proxy for general gun availability -- fall significantly.

Similarly, Illinois had a background check requirement before 1994, so the local gun market was not affected by the passage of the Brady Act, but gun trace data shows that criminals simply switched from smuggling guns from out of state to buying them illegally in state.

There are no comprehensive records kept of incidents where guns are used in self-defense, so the only way to know is to ask people. Data from the National Crime Victimization Survey suggest that a gun is used in self-defense about 60,000 to 120,000 times each year. Several other surveys confirm this estimate. By comparison, each year about a million violent crimes involve guns. This means guns are used to commit a crime about 10 times as often as they are used for self-defense.

A few surveys in the early 1990s suggested that there are millions gun self-defense incidents each year, but there are very good reasons to believe that these estimates were improperly calculated and these numbers are way off, more than 10 times too high. If the numbers really were this high, this would imply that pretty much every gunshot wound in America is the result of somebody protecting him or herself.

Even among the more accurate surveys, according to a panel of criminal court judges who reviewed survey respondents' stories, about half the time the gun use was "probably illegal," even assuming the gun itself had been purchased legally.

The crux of this question is whether most homicides are planned, or whether killers more often confront their victims with no clear intention. In the second case, adding a gun could result in a fatal shooting that would otherwise have been avoided.

The evidence that weapon does matter goes back decades. In 1968, Franklin Zimring examined cases of knife assaults versus gun assaults in Chicago. The gun attacks were five times more deadly. Moreover, the two sets of attacks were similar in all other dimensions: age, sex, race, whether the victim knew the assailant beforehand, and so forth. A few years later, he repeated his analysis, this time comparing small and large caliber guns. As expected, the victim was much more likely to die from larger caliber guns.

Although it is surely true that a determined killer cannot be stopped by the absence of a gun, this type of evidence indicates that many homicides are unplanned. The outcome depends, at least partially, on the weapon at hand. In that restricted sense, guns do kill people.

This is not an easy question to answer, because crime rates can decline for a wide range of reasons. For example, violent crime rates declined sharply all across the country in the mid-1990s, regardless of whether a given area had tightened its gun laws. So based on a naive interpretation of the numbers, any attempt at reducing gun violence in 1995 would have appeared successful by 1998. Then there is the problem of comparing different states or cities: Circumstances differ, and what works in Memphis may fail in Detroit.

Nonetheless, there are some plausible methods for isolating the different factors, using comparison groups or other controls. The most thorough summary is a 2008 meta-analysis where the authors reviewed every prior American gun violence reduction study, examining both the reported effectiveness and the strength of the statistical evidence. Here are some approaches that don't seem to work, at least not by themselves, or in the ways they've been tried so far:

Stiffer prison sentences for gun crimes.

Gun buy-backs: In a country with one gun per person, getting a few thousand guns off the street in each city may not mean very much.

Safe storage laws and public safety campaigns.

We don't really have good enough evidence to evaluate these strategies:

Background checks, such as the Brady Act requires.

Bans on specific weapons types, such as the expired 1994 assault weapons ban or the handgun bans in various cities.

These policies do actually seem to reduce gun violence, at least somewhat or in some cases:

More intensive probation strategies: increased contact with police, probation officers and social workers.

There is no obvious solution here, and there's a huge amount we still don't know. But it's possible that combinations of these policies, or variations in a different context, might work better. For example, background checks would probably be more effective if they were also applied to private sales. Also, of course, this list does not include policies that have not yet been tried.

There is no way to know whether the recent White House proposals will be effective in reducing gun violence. How can there be, when it's so difficult to assess the actual policies that have already been tried, let alone vague plans? But the White House proposals do at least plausibly target several components of the gun violence problem.

Probably the most significant proposal is the idea of requiring background checks for gun sales between private individuals, not just from licensed dealers (with some exceptions, such as transfers within a family). Private sales are currently the main way guns move between legal and illegal owners. However, the White House has yet to specify how a private seller would perform such a check.

There is less evidence for the effectiveness of banning assault weapons and large-capacity magazines. During the 1994-2004 assault weapons ban, the use of assault weapons in crimes fell, but use of large-capacity magazines increased. This is thought to be largely due to the huge number of already-owned LCMs, which were exempt from the ban on manufacturing and sales. If the new law does not address the LCMs already in private hands, it may be decades before it has any real effect.

Removing legal restrictions that prevent the Centers for Disease Control and other agencies from tracking and researching gun violence is also a sensible idea, and follows a long history of calls from scientists (see: what don't we know).

How does the U.S. compare to other countries?

The U.S. has one of the highest rates of violent crime and homicide, per capita, of any developed country. According to 2008 figures compiled by the United Nations Office on Drugs and Crime, the U.S. homicide rate for 2010 is 5.1 per 100,000 people. Only Estonia's is higher, at 6.3. The next most violent country is Finland, which has a homicide rate of 2.5, half that of the U.S. The remaining 28 developed countries are even lower, with an average of 1.1 homicides per 100,000 people.

But many less developed countries have much higher homicide rates -- for example Columbia (35.9), South Africa (36.8) and Sudan (24.2). This analysis uses the 2012 IMF list of developed countries.

The U.S. also has the highest rate of civilian gun ownership in the world, by far. The best data is from the 2007 Small Arms Survey, which notes:

With less than 5% of the world's population, the United States is home to roughly 35-50 per cent of the world's civilian-owned guns, heavily skewing the global geography of firearms and any relative comparison.

A lot! We lack some of the most basic information we need to have a sensible gun policy debate, partially because researchers have been prevented by law from collecting it. The 2004 National Research Council report discussed above identified several key types of missing data: systematic reporting of individual gun incidents and injuries, gun ownership at the local level, and detailed information on the operation of firearms markets. We don't even have reliable data on the number of homicides in each county.

For such sensitive data sets, it would be important to preserve privacy both legally and technically. There have been recent advances in this area, such as anonymous registries. But the Centers for Disease Control, the main U.S. agency that tracks and studies American injuries and death, has been effectively prevented from studying gun violence, due to a law passed by Congress in 1996.

This may not technically rule out basic research, but scientists say it has made the issue so sensitive that key funding agencies will not support their work. They point to grant data as evidence of an effective ban. The White House has recently proposed lifting these research restrictions (see above)